Hexagonal SiC has a very high heat conductivity, and both electrically conductive and insulating substrates are available. Its lattice constant and coefficient of thermal expansion are relatively close to those of AlN and GaN-based Group III nitrides. Another characteristic of hexagonal SiC is that it is a hexagonal crystal and possesses polarities, as to Group III nitrides.
There are high expectations for the realization of a technology for growing high-quality crystals of Group III nitrides on SiC for applications relating to a buffer layer for the formation of a GaN-based device structure on an SiC substrate, or relating to Group III nitride/SiC heterojunction devices. It has been difficult to grow a high-quality Group III nitride layer on SiC because of the mismatch of the stacked structure of SiC and Group III nitrides in the c-direction, or the so-called polytype mismatch. Namely, 4H—SiC and 6H—SiC, which are representative of hexagonal SiC, have structures with 4- and 6-monolayer periods, respectively, in the c-axis direction, while AlN and GaN, which are Group III nitrides, have 2-monolayer periods in the c-axis direction in a structure referred to as the wurtzite structure.
In order to solve this problem, it has been proposed to make the SiC substrate surface a flat plane without any steps, or to control the height of the steps on the SiC substrate surface to be common multiples of the stacking periods of SiC and the Group III nitride. For example, a technology has been proposed whereby a SiC substrate surface is subjected to HCl gas etching so as to form a SiC surface with the aforementioned features, which is followed by the growing of an AlN layer (see Non-patent Document 1: Norio Onojima, Jun Suda, and Hiroyuki Matsunami, “Molecular-beam epitaxial growth of insulating AlN on surface-controlled 6H—SiC substrate by HCl gas etching,” Applied Physics Letters, Vol. 80, No. 1, (2002) p. 76-78, for example).